Rotational molding is a method for manufacturing hollow articles such as containers, from a thermoplastic material. The particles of said thermoplastic material in the form of liquid, powder or pellets, are provided within a hollow mold and the mold rotates in a heated environment until the thermoplastic material is molten within all the inner surface of the mold.
There are several kinds of machines, from the small and simple ones, to the ones with large rotation diameters with sophisticated controlling electric system. Its selection is made based on certain parameters to consider, as the size of piece, production rate, planned investment.
There are also different rotomoulding systems; the simplest known as stationary or quiescent to which the the mould carriers enter and come out; others with a central rotary unit or roundabout, with three, four or more arms that move among the different work stations (load, heating, cooling, load/unload). For pieces of large dimensions, the rotary looper-like machines are used.
The traditional rotomoulding process is made in open flame, with the disadvantage that the thickness of the walls of the resulting article is not homogeneous, as the heating of the mold is irregular. On the contrary, the processes of rotomoulding in furnace, allow the achievement of great quality articles, with homogeneous thickness walls. However, when it comes to manufacturing articles with multiple layers, the use of a furnace is impractical, as once the first layer is formed, the next load of material to be molten, generally implies the removal of the mold from the furnace. Alternatively, in order to avoid the removal of the mold from the furnace, a gate from the furnace is opened, and the thermoplastic material is introduced, either by means of a bin or box commonly named “drop-box”, or by means of any spray device. Generally, the feeding stage, especially with the use of “drop-box”, is commonly performed manually, and even in some cases, it comes necessary to suspend heating, as the material within the “drop-box” may be plastified before being poured into the mold. The above implies delays in the overall process, losses of heat from the furnace and thus, greater energy expenditure. The feeding stage becomes further limiting, when it comes to manufacturing several articles of several layers in a single cycle.
In the prior art there are several devices, which serve for introducing materials to the rotomoulding furnace. U.S. Pat. No. 4,687,531 from Potoczky, describes a device for layering structures with uniform walls that are made by rotation and heating in a mold, the mold being in an horizontal position, the device of the prior art, in order to achieve its objective, uses a device for introducing the material with a material sprinkler, which, by means of compressed air, spreads the material due to the use of an arm in the edge that is inserted into the mold, this arm may be moved at the horizontal axis and the part wherein the materials exit has a 90° movement, the mold is in a furnace which heats to a desired temperature. This device has the disadvantage that, at the moment of placing the material, there are some places wherein the provision of the material is not made, further, it requires involving a person that has the skills for handling the arm and providing the materials in a uniform manner, however, the device of the invention uses molds, which may be immediately reutilized for introducing the same material to the next mold, this is advantageous in a production line of more than one mold, further, in order to achieve the layering of two or more layers in the device of the prior art, it is necessary to provide another arm with any other material which, by the way, is not suggested in the same; finally, the arm has no cooling means, which eases the introduction of a different material for the layering of two or more layers.
U.S. Pat. No. 3,744,951 from Szatkowski, describes a system for manufacturing containers, the system is provided with a furnace with containers in a roundabout shape, has different stations in which the molds are passed through each layering stage, one station has an arm for introducing the material, the arm is introduced within the molds for spreading the material as a powder and then be removed to continue with the rotomoulding; although it is an arm that enters and exits the mold, has the disadvantage that the same is made by means of sprinklers, thus, a number of supporting elements are used; further, for moving the arm, such arm has to be moved with all the basement set, so the energy consumption is relevant; further, for the layering of two or more containers, it is necessary that each mold moves to the arm position, which entails to manufacturing a minor number of containers per time unit; when using a roundabout, it may only feed a single container, and for the accomplishment of two or more layers, it would be necessary to have two or more arms with material, which would increase the cost of production, as the arm may only use one single material. The disadvantage of this device is that its maintenance is higher, and that it may only introduce the material one at a time. The cleansing on each introduction is possible only once the line of production is stopped.
Publication No. WO 2008/133535, shows a furnace wherein molds of different shapes are introduced and in wherein the horizontal axis of the molds is rotated for manufacturing pieces; this device has a gate in which there is a window through which a tube is introduced, said tube, by means of pressure, spreads the powder of the material into the inner part; once the material contacts the heated mold, it forms the desired piece, however, this furnace only makes one single piece at a time, and there is no mention of the material being deposited and that such material, due to the rotation, is homogeneously distributed to the mold. The material is not only deposited within the mold, it is also blown, so that afterwards, by means of rotation, it may be distributed to the mold. The disadvantage of this device is that, for the achievement of two or more layers, it would be necessary to have an arm for each material, as such arm may not be cleaned at the moment of manufacturing the product, and it would be necessary to have two tubes for introducing the materials, increasing, at the same time, the production cost.
As a further alternative, U.S. Pat. No. 4,632,654 describes the use of a furnace for rotomoulding that is attached to a cooling chamber, wherein the molds are carried from the furnace to the cooling chamber and, once loaded with the second layer material, they are again directed to the furnace and later to the cooling chamber, and so on. This system is complex, as it requires a device for carrying the molds from one chamber to another, and the time of production is considerably elongated.
Until now, no process allows for the manufacturing of multilayer articles in furnace through an automated cycle, highly efficient, with great volumes of production and without energy losses, wherein the bins for the introduction of the material to the molds are used, and that those same bins may be used in the same production cycle for providing the forming material of the first layer to the molds; those same bins, once cleaned, are used for providing the next layer of material, and so on, until the the completion of the desired number of layers is achieved.